1. Field of the Invention
This invention relates to a power source for deriving from a DC power source desired DC output, sine-wave AC output, etc. having voltages different from the voltage of the DC power source, and more particularly to a power source of the type using a high-frequency phase difference control and possessing an ability to cause the transfer of electric power between the power source side and the load side to be made through a high-frequency transformer and to effect control of the output voltage by subjecting the power source side (input side) and the load side circuits to ON-OFF control with a certain phase difference, where the power source side and the load side are required to be isolated from each other in terms of DC.
This device can be advantageously used as an uninterruptible power source, a device for controlled drive of a motor, a DC constant-voltage power source, or a constant-current device, for example.
2. Description of the Prior Art
Most, if not all, recent data processing devices are unable to tolerate an interruption of power supply even momentarily. In these circumstances, uninterruptible AC power sources which are provided with an inverter and enabled to derive a sine-wave AC output from a DC power source by switching the DC power source have been finding growing utility in actual applications. In this case, more often than not there arises a need for keeping the sine-wave AC output isolated from the DC power source in terms of DC.
FIG. 1, FIG. 2, and FIG. 3 are block diagrams illustrating configurations of conventional AC power sources of the type deriving a sine-wave AC output by the ON-OFF control of a DC power source. In these diagrams, like symbols denote identical or similar parts.
In the power source of FIG. 1, the electric power from a DC power source 1 is converted into an AC having the frequency of a commercial power supply by a switching device 2 which is adapted to derive a sine-wave AC output by the On-OFF control of a switching element incorporated therein, and the AC power thus produced is supplied via a transformer 3 to a load 4 ("Principles of Inverter Circuits," pp 310 to 318, written by B. D. Bedford et al. and published by John Wiley & Sons, Inc.).
This power source has a disadvantage that since the AC power of a low frequency (50 or 60 Hz) is transmitted via the transformer 3 to the load, 4, the transformer 3 suitable for these frequencies occupies a large space and is heavy.
In the power source of FIG. 2, the electric power from the DC power source 1 is converted into an AC of high frequency by a DC-AC converter 5 and then supplied via a high-frequency transformer 3H to a rectifier 6. The DC out having the rectifier 6 is converted into a sine-wave AC of the frequency of a commercial power supply by the same switching device as used in the power source of FIG. 1 and the produced AC is supplied to the load 4.
In the power source of FIG. 2, since the electric power is transmitted in the form of a high-frequency AC via the transformer 3H, the high-frequency transformer 3H is not so bulky as mentioned above. On the other hand, however, this power source has a disadvantage that since the rectifier 6 is an unidirectional element restricting the flow of electric power to only one direction and the reactive power is not regenerated in the power source and tends to give rise to a surge voltage where the load is other than a pure resistance. The power source requires a measure for preventing the adverse effect of the reactive power which significantly complicates the configuration of circuit.
In the power source of FIG. 3, the electric power from the DC power source 1 is converted by a switching device 2P into a high-frequency AC which is pulse-width-modulated in the form of a full-wave rectified sine wave and supplied to the high-frequency transformer 3H. The high-frequency AC secondary output of the transformer 3H is rectified by the rectifier 6 to be converted into a full-wave rectified sine-wave AC and then is converted further by an unfolding circuit 7 such as a bridge circuit into a sine-wave AC with the commercial frequency (an article written by Alan Cocconi et al. under the title of "High Frequency Isolated 4 KW Photovoltaic Inverter For Utility Interface" at pages 325 through 345 in "ADVANCES IN SWITCHED-MODE POWER CONVERSION," Vol. III, published by TESLA Co.).
The power source of FIG. 3, similarly to that of FIG. 2, enjoys freedom from undue volume and weight of the transformer 3. Similar to the power source of FIG. 2, however, it has a disadvantage in that it requires a measure for preventing the otherwise inevitable occurrence of a surge voltage due to the reactive power which complicates the configuration of circuit.